Embodiments of the invention relate generally to X-ray tubes and more particularly to an apparatus for improved focusing control and increased useful life of the tube.
Typically, in computed tomography (CT) imaging systems, an X-ray source emits a fan-shaped beam or a cone-shaped beam towards a subject or an object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” may be used to include anything that is capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the X-ray beam by the subject. Each detector element of a detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis. The data processing system processes the electrical signals to facilitate generation of an image.
Generally, in CT systems the X-ray source and the detector array are rotated about a gantry within an imaging plane and around the subject. Furthermore, the X-ray source generally includes an X-ray tube, which emits the X-ray beam at a focal point. Also, the X-ray detector or detector array typically includes a collimator for collimating X-ray beams received at the detector, a scintillator disposed adjacent to the collimator for converting X-rays to light energy, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom.
Currently available X-ray tubes employed in CT systems fail to control the level of electron beam intensity to a desired temporal resolution. Several attempts have been made in this area by employing techniques such as controlling the heating of the filament, employing Wehnelt Cylinder gridding that is typically used in vascular X-ray sources and by employing an electron acceleration hood on the target of the X-ray tube to control electron beam intensity. Also, currently available microwave sources include an electron gun that includes a focusing electrode, such as a Pierce electrode to generate an electron beam. These electron guns typically include a grid to control a beam current magnitude via use of control grid means. Unfortunately, the energy and duty cycle of the electron beam makes the introduction of an intercepting wire mesh grid difficult since the thermo-mechanical stresses in the grid wires are reduced when the intercepted area of the electron beam is minimized. Furthermore, rapidly changing the electron beam current prevents proper positioning and focusing of the electron beam on the X-ray target. Modulation of the electron beam current from 0 percent to 100 percent of the electron beam intensity changes the forces in the electron beam, due to changes in the space charge force resulting in change in the desired electro-magnetic focusing and deflection.
In addition, current X-ray tubes have limitations with regard to the emission current that can be utilized in the X-ray tube. The primary reason for this is that higher emission currents cause the emitter in the X-ray tube to fail prematurely as a result of the increased temperature leading to accelerated burnout of the emitter at these emission current levels.
Hence, it is desirable to control focus and position of the electron beam on a same time scale to preserve image quality, imaging system performance, and durability of the X-ray source. It is also desirable to increase the emission current capable of being utilized in the X-ray source/X-ray tube without compromising the useful life of the X-ray tube.